Communication system



Aug. 6, 1946.

COMMUNICATION SYS TEM Filed J u1y 16, 1942 INVENTOR R. L/ L. HARTLEY BVWWKW ATIQRNEV R. v. L. HARTLEY 2,405,211 I Patented Aug. 6, 1946COMMUNICATION SYSTEM Ralph V. L. Hartley, Summit, N.

J assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application July 16, 1942, Serial No.451,139

10 Claims. 1

This invention relates to communication systems and particularly to asignaling means for use therein.

An object of the invention is to provide a novel signaling device forcontrolling the switching equipment in a central office.

Another object of the invention is to provide an improved calling devicefor controlling the central office switching mechanism at a higher rateof speed with consequent reduction in the subscriber's signaling time.

A further object is to p ovide a sig a i vice in which a single impulseis transmitted for each digit.

With the above objects in view, an embodiment of the invention disclosesa signaling device which, when operated, transmits to the central ofllcea characteristic signal pulse of variable duration dependent upon thedesired digit instead of the customary series of equal impulses for eachdigit dialed.

The signaling device contemplated for use in this invention is of theelectromagnetic type and comprises an electromagnetic core havin aprimary coil connected to a telephone line in series with a manualcircuit breaking switch and a secondary coil connected to any one of tenvalues of resistance through a series of ten digital keys, each operatedkey connecting a predetermined resistance thereto, key I inserting themaximum resistance and key the minimum resistance into the secondarycoil circuit. A movable element or armature of the electromagnet isprovided for conditioning the circuit breaking switch, which in itsinoperative position is closed because of the action of a biasing springattached to the armature. When a telephone subscriber raises his handsetfrom the mounting, the primary coil thereof is energized and thearmature in turn is attracted to a position which allows freedom ofmovement for the circuit breaking switch. The subscriber in proceedingwith his call'initiates an impulse of current by actuating one of thedigital keys, which opens the line circuit by means of the circuitbreaking switch and which connects a resistance, the value of which isdependent upon the key punched, to the secondary coil of theelectromagnet. Interruption of the primary coil induces an electromotiveforce in the secondary coil which sets up a magnetic flux sufiicient tohold the spring biased armature attracted for a period corresponding tothe digital key operated. Retraction of the armature completes theimpulse by closing the cir- 'cuit breaking switch, which conditions theline circuit for the next impulse of current to be transmitted by thesubscriber. Duration of the signalin impulse is variable and isdetermined by the amount of resistance connected across the secondarycoil 21 by the actuated digital key, actuation of key I providing theshortest impulse and actuation of key 0 providing the longest impulse.

The invention can be more readily understood by reference to thefollowing detailed description in connection with the drawing whichshows one embodiment thereof.

There is shown diagrammatically a telephone subscribers station 9 of theusual type and a signaling device In connected by a pair of line wiresII to a central office switchin mechanism. Signaling device I 0comprises a U-shaped electromagnetic core 12 having a pair of projectingpoles I3, I 4, an armature l5 of soft iron rotatably disposed betweensaid poles by means of a shaft H, and an arm l8, which is secured toarmature IS in any suitable manner, having near its free extremity aninsulated finger l9. Arm I8 is biased to its normal position, as shownin solid, by a retractile spring 2| which is secured at one end to arm[3 and at the other end to a stud 22. Stop pin 23 restricts the biasingaction of spring 2| upon arm l8. Armature I 6 provides a closed magneticcircuit in conjunction with poles l3, M of core l2 when arm [8 is in itsoperated position, as shown dash-dotted, under the influence of the pullexerted by the magnetic flux in core l2, which will be explained ingreater detail hereinafter. Around core I2 is wound a primary coil orwinding 24, one end of which is connected to one side of line H and theother end of which is connected through a back contact 25 of switch 26to the other side of line ll. Switch 28 may be of the customary springtype of manual circuit breaker, which can be rapidly snapped from aclosed to an open circuit basis or from an open to a closed circuitbasis, and is so arranged that it is closed when armature I6 is releasedby the magnetic flux ceasin to overcome the pull of biasing spring 2|,as will be described hereinafter.

A secondary coil or winding 27 is also wound around core l2, one end ofthe coil being connected to a common base plate 28 of key-set 29 whilethe other end of coil 21 is connected to one end of resistance 3|, whichis subdivided serially into ten sectional resistances of variable value,as described hereinafter, by connecting taps 32 thereof to correspondingkey levers I, 2, 3, 4, 5, 6, 1, 8, 9, i! of key-set 29. The lowestresistance value is connected to key lever 0, the

next higher resistance value is connected to lever 5. and in thissequence the resistance sections are connected until the entireresistance 3| is connected to lever I. Resistance 3I, as illustrated, isintended to be merely representative and it is of course understood thatindividual resistances of proper value may be so used. Each of levers Ito U, inclusive, is provided with a button 33, a contact 34 forengagement with a contact 35 secured to common base-plate 28, and aturnedover portion 36 for actuating a movable insulated plate 31, whichhas a plunger 38 attached thereto. A cam 39, formed n plunger 38 nearthe free end thereof, is arranged for engagement with the free end of apivotal bar 4|, insulated projection 42 on the under side andintermediate the ends thereof for actuating switch 26. Bar All is biasedto the upward posi-. tion by spring-43, stop 44 limiting the extent ofthe upward travel and stop 46 limiting the extent of the downward travelthereof. The lower end of plunger 38 engages a leaf spring 41, which isattached to an insulated support 48 in any suitable manner and which isadapted to maintain plunger 38 in an upwardly biased condition. Thedetailed construction of key-set 29 is not further illustrated asswitches of this type are of known design.

For operation of key-set 29, pressure on any of the series of buttons 33first causes engagement between contacts 34 and 35 and then through thecooperation of plunger cam 39, bar M and projection 42, movableinsulated spring 49 of switch 26 is snapped open to the position shownin dot-dash lines. Bar M is restricted in its downward movement by stop46 so that cam 39 continues to a position underneath the free end of bar4|, which thereupon because of the tension of spring 43, is retracted toits normal position, thus clearing the way for restoral of spring 49, aswill be hereinafter described. Release of pressure on button 33 enablesleaf spring 41 to restore plunger 38 to its normal position as shown.

For a better understanding of the invention, the principles applicableto electromagnetic relay operation will be compared with thoseapplicable to the device disclosed herein. It is well known in telephonerelay practice that relays may be made slow in operating and releasingby causing a current to be induced which in turn produces a magneticfield in opposition to the original changing field and prevents theresultant magnetic field of the relay from building up or dying down asrapidly as the current in the relay coil. This is done by means of ashort-circuit winding on the relay in which currents are induced toprevent the current in the regular winding from building up its field ordropping off its field as rapidly on opening the circuit. Thus inoperating, the full strength of the field, which is required to pull upthe armature of the relay, is not obtained until after a definite lapseof time.

The above principle is applied to this invention wherein primary winding24 correspondsto the regular winding of a relay and secondary winding 21corresponds to the short-circuited winding of the relay. Whilevariations in the operating and releasing periods of the relay areobtained mechanically by varying the gap between the armature and corethereof, variations in the release of armature I6 of signaling device I0are obtained by connecting resistances of variable amounts to secondarywinding 21 thereof through operation of the corresponding digital keysof key-set which has an 29. The interval during which line II is opened,following operation of a digital key, depends upon the time constant ofthe circuit comprising winding 21, the sectional value of resistance 3|con nected to winding 21, and on the fraction of its initial value towhich the magnetic flux must fall in order for finger I9 to close switch26. The flux may be kept reasonably constant by designing core I2 sothat it is always saturated by the line current. The time constantvaries inversely as the resistanc value connected to winding 21.Therefore by providing resistance values, which correspond to thevarious digital keys, as submultiples of the overall resistance value,the intervals can be made proportional to the digital numbers. In otherwords, digital key I, which inserts the maximum resistance, produces theminimum open circuit condition in line II; key 2, which introducesone-half of the resistance of key I, produces twice the open circuitperiod of key I; key 3, which introduces one-third of the resistance ofkey I, produces three times the open circuit period of key I; and inlike manner continuing to key 0, which inserts one-tenth of theresistance of key I and produces ten times the open circuit period ofkey I.

Thus a novel and useful method of indicating a number is providedwhereby the subscriber controls the duration of an interval which isobservable at a central office and which is convertible thereat throughwell-known means into a numerical indication. A practical advantage ofthis method lies in the-fact that if the interval can be made short, theappropriate signals can be transmitted very quickly, thereby reducingthe subscribers signaling time.

The detailed operation of signaling device I0 will now be described. Letit be assumed that the subscriber wishes to initiate a call and thatline circuit I I is open at handset switch contact I5 by virtue of thehandset (not shown) being on its mounting, switch 26 is closed, arm I8is biased to its upper position by spring 2|, and armature I6 beingattached to arm I8 is likewise biased from that of a magnetically closedcircuit in conjunction with poles I3, I4. The subscriber in removing thehandset from the mounting es tablishes, in the customary manner, adirect current circuit including primary winding 24 in series withcontact 25 of switch 26, handset switch contact I5, and the centralofiice'switching mechanism over line II. The resultant current flowthrough primary winding 24 sets up in core. I2 a magnetic flux, which issufiicient to rotate armature I6 and arm I8 in opposition to spring 2I,so that the opposite faces of armature I6 are approximately parallelwith the faces of poles I3, I4, respectively, thereby closing themagnetic path therethrough. Finger I9 is also rotated with arm I8, theoperated positions of armature I6, arm I8, and finger I9 being shown indotdash lines. While in the operated position, finger I9 is thus clearof movable spring 49 of switch 26. The subscriber next pushes a button33 of any one of the digital keys of key-set 29, say key lever B, whichfirst engages contacts 34, 35 connecting resistance element 53 ofresistance 3I across secondary winding 21, and then actuates plunger 38.Movable spring 49 is snapped to its open circuit position by plunger38in combination with cam 39, bar 4I, and projection 42, as hereinbeforedescribed, and as shown by the dotdash lines, whereby the above-tracedline circuit is opened. The line current stops abruptlybut it is to benoted that armature I6 does not immediately retract to its biasedposition under the tension of spring 2|. Instead, a value of currentsubstantially equivalent to the line current is set up in Winding 21,through induction from primary winding 24, which maintains the magneticflux in core I2 constant for the moment. This prevents a surge ofvoltage across primary winding 24. The current in secondary-winding 21and magnetic flux in core l2 then decrease exponentially at a ratedetermined by the above-described time constant of the secondarycircuit. When the magneti flux in core [2 falls to a value where itspull on armature I6 is overcome by the pull of biasing spring 2|,armature I6 is retracted to its biased position and while returning tothis position causes finger l 9 to strike spring 49, which snaps backand closes the line circuit at contact 25. The line current immediatelyrises to a value determined by the resistance of primary winding 24, themagnetic flux in core l2 remaining unchanged due to an equivalentcurrent set up in secondary winding 21, which then dies out and permitsthe fiux tobuild up to the value corresponding to the line current. Thesubscriber then releases button 33 which opens the circuit of secondarywinding 21 by separating contact 34 from contact 35 and which permitsplunger 38 to be restored to its normal position by leaf spring 41, ashereinbefore described. The signaling device I is now in condition forthe next number or series of numbers that may be transmitted, asrequired, in the above-described manner. It is to be noted that button33 is kept in the operated position during the impulse interval in orderto prevent the subscriber from prematurely opening secondary winding 21before the entire pulse had been transmitted.

While the features of this invention have been disclosed with referenceto a specific embodiment only, it is, of course, understood that variousmodifications may be made in the details of construction withoutdeparting from the scope of this invention as defined in the appendedclaims.

What is claimed is:

1. In combination, an electromechanical switch, a magnetic circuit insaid switch, means for producing flux in said magnetic circuit, acontrol circuit for said switch, means in said control circuit includinga plurality of impedance elements properly proportioned with respect toeach other for varying correspondingly the flux flowing in said magneticcircuit, and means for selecting any one of said impedance elements andfor concurrently rendering said flux producing means ineffective,whereby the time of operation of said switch is predetermined for afixed interval.

2. In combination, an electromechanical switch, a magnetic circuit insaid switch, means for generating a flux in said magnetic circuit, acontrol circuit for said switch, means in said control circuit includinga plurality of impedance elements properly proportioned with respect toeach other for varying correspondingly the flux flowing in said magneticcircuit, and means for selecting any of said impedance elements and forconcurrently rendering said flux generating means ineffective, wherebythe operating time of said switch is predetermined for a plurality offixed intervals of variable duration.

3. An electromechanical switch comprising an electromagnet, an armatureassociated with said electromagnet, a primary inductance coil forgenerating a magnetic field of a fixed value wound around saidelectromagnet, a time control circuit including a series of resistancesand a secondary inductance coil wound around said electromagnet forcontrolling operation of said switch, and means for establishing thetime constants of said control circuit at a plurality of fixed values,said means including a keyset operable to select any one of saidresistances and to concurrently open said primary coil, whereby magneticfields of different definite durations for modifying the time ofoperation of said switch are set up.

4. A signal transmitter for transmitting variable length pulses over aline circuit comprising, in combination, an electromagnet having a core,primary and secondary windings carried by said core and an armature,pulsing contacts serially including said primary winding in the linecircuit to effect the operation of said armature, manually operablemeans for operating said contacts to disconnect said primary windingfrom said line circuit, the operation of said contacts initiating thetransmission of a pulse over the line circuit and the deenergization ofsaid electromagnet. means also responsive to the operation of saidmanually operable means for including a resistance in series with saidsecondary winding concurrently with the operation of said contactswhereby the deenergization of said electromagnet is delayed for aninterval of time, the length of which is determined by the magnitude ofthe said resistance, and means controlled by the armature of saidelectromagnet when deenergized for restoring said contacts, therestoration of said contacts terminating the transmission of the pulse.

5. A signal transmitter for variable length pulses over a line prising,in combination, a normally closed contact whose opening determines thestart of a pulse and whose reclosure determines the end of a pulse, andmeans for controlling said contact comprising an electromagnet havingtwo inductively related windings mounted on the core thereof, one ofsaid windings being serially included in the line circuit by saidnormally closed contact, manually operable means for operating saidcontact to open circuit condition and for concurrently closing saidother winding through a resistance, the magnitude of which determinesthe length of the pulse transmitted over the line circuit, whereby themagnetic condition of said electromagnet which prevailed prior to theoperation of said contact is maintained through the inductiveinteraction of said windings for an interval of time commensurate withthe value of resistance connected to the said other winding incident tothe operation of said manually operable means, and means including anarmature controlled by said electromagnet for reclosing said contact atthe end of the said interval of time.

6. A time duration signaling device for a telephone switching systemcomprising means for establishing a predetermined line signalingcondition of selectable duration, said means comprising, in combination,a line circuit, a local selecting circuit inductively coupled to saidline circuit, a contact maintaning said line circuit in closedcondition, manually operable means for operating said contact to opensaid line circuit, and means controlled by said inductively coupledcircuits for operating said contact to close said line circuit.

7. In an electromagnetic signal transmitter, the combination with a linecircuit of an energy storage element serially connected with said linecircuit, a switch for interrupting said line circuit, flux generatingmeans inductively coupled with transmitting circuit comsaid storageelement for controlling said switch, a series of resistances for varyingthe impedance of said generating means, digital keys for operativelyconnecting said resistances to said flux generating means and foroperating said switch, and flux responsive means held actuated for aperiod corresponding inversely to the value of said resistance connectedto the generating means circuit, said flux responsive means effective toretract said switch to itsoriginal position at the end of said period.

8. A device for producing impulse signals in an electrical circuitcomprising a switch contact, an electromagnet having an armature forclosing said contact, an energizing circuit for said electromagnetincluding a primary winding and said contact, a control circuit for saidelectromagnet including a secondary winding and a resistance adaptablefor connection in series with said secondary winding, said resistancebeing subdivided into predetermined elements, and separate means forconnecting any of said resistance elements with said secondary windingand for concurrently opening said contact to initiate an impulse signal,whereby said armature after a predetermined time, dependent upon theresistance element selected, recloses said contact to complete saidimpulse signal.

9. A device for producing impulse signals of variable duration in anelectrical circuit comprising an electromagnet having an armature, an

energizing circuit for said electromagnet including a primary windingand an impulse contact, a control circuit for said electromagnetincluding a secondary winding and a resistance adaptable i or connectionin series with said secondary winding, said resistance being subdividedinto predetermined elements, and a key-set for connecting any of saidresistance elements with said secondary Winding and for concurrentlyopening said contact to initiate an impulse signal, whereby saidarmature is efiective to close said contact to complete said impulsesignal after a predetermined time, dependent upon the resistance elementselected.

10. An impulse signaling device comprising an impulse switch contact,manual means for opening said contact to initiate an impulse,electromagnetic means for reclosing said contact, said electromagneticmeans comprising a primary winding in series with said contact, a springbiased armature under control of said winding for closing said contact,and a time control circuit including a secondary winding and a series ofresistance elements for controlling the time of reclosure of saidcontact, said manual means also operative for connecting a particularone of said resistance elements in circuit with said secondary windingconcurrently with the opening of the contact, whereby the length of saidimpulse is predetermined for a fixed interval.

RALPH V. L. HARTLEY.

